This invention relates to xenon-enhanced computerized tomography (CT). More particularly, this invention relates to apparatus for measuring xenon gas concentrations useful in derivation of cerebral blood-flow (CBF) estimates by xenon-enhanced computerized tomography.
A technique has been developed in recent years whereby images mapping the level and efficiency of cerebral blood flow can be generated on a CT scanner. The technique involves the inhalation of a xenon gas mixture by a patient during a series of CT scans. The xenon gas is absorbed into the patient's bloodstream and diffuses into surrounding tissue altering the X-ray absorption factor of the blood and diffused tissue. This change enhances the affected areas on the normal CT images. Over a series of scans, the rate of change in a region of tissue is a direct indication of the blood flow and diffusion rate in that region. Using data obtained over a series of scans it is possible to reconstruct, using known techniques, a blood-flow image where gray scale is proportional to blood-flow rate. These images are used to diagnose stroke cases and to screen potential stroke victims, as well as many other diagnostic procedures still in developmental stages.
The image reconstruction process requires that the concentration of xenon in the bloodstream be known at the time of each CT scan in the series. While various methods have been developed for measuring xenon concentration, all non-invasive methods derive bloodstream concentration by measuring the patient's end-tidal concentration in expired air. Xenon concentrations in end-expired gas are known emperically to be equivalent to xenon concentrations in arterial blood.
Most commonly, measurements of end-tidal xenon concentrations have been made by one of three basic methods. The subtraction method is based on the assumption that in a denitroginated subject the exhaled gases are predominantly oxygen, carbon dioxide, water vapor and xenon, and that, therefore, the concentration of xenon can be obtained by subtracting the other three components from atmospheric pressure. This technique is acceptable provided oxygen and carbon dioxide concentrations can be measured with instruments having the requisite sensitivity and response speed. However, due to the fact that the subtraction method assesses xenon concentration in expired gas indirectly, it fails in patients who have not been completely denitroginated. In addition, because the effects of pure oxygen inhalation on blood flow during denitrogination, it is preferable to perform blood-flow studies without that process. The end-tidal xenon concentration can be measured directly utilizing either a pre-calibrated mass spectrometer or a thermal conductivity detector. The mass spectrometer is the most accepted method for measuring xenon concentrations. However, the mass spectrometer is an expensive and complex device to operate. Conventional thermal conductivity detectors have been used successfully to measure xenon concentrations. In this type of system, the detector monitors the concentration of xenon in expired gas continuously. This system, used conventionally, provides the required accuracy but is limited by the relatively slow response time of the detector. At breath rates over approximately 18/minute or concentrations of over 40 percent, the detector cannot track the xenon concentration waveform accurately.
It is, therefore, an object of the invention to provide a low-cost, efficient and simple technique for measuring xenon concentrations in xenon CBF studies.
It is another object of the invention to provide an apparatus using a thermal conductivity detector for measuring xenon concentration in CBF studies which is independent of breath rate and xenon concentration.
It is still another object of the invention to provide an apparatus using a thermal conductivity detector for measuring xenon concentration in CBF studies wherein the detector responds substantially only to actual change in concentration from breath to breath.